Intermeshing co-rotating twin-screw extruders as known in the art conventionally utilize a number of different elements mounted along the respective extruder shafts according to a sequence of process functions that the extruder is to perform.
In general, the screws in such an extruder include a number of transporting (forward-conveying) screw elements designed to accept the plastic material and additives and to convey them to a special section of the extruder dedicated to transforming the plastic material into a thermally homogeneous compound melt including the additives. This special section of the extruder conventionally includes a number of kneading elements, usually in the form of blocks or discs, designed to impart high energy per unit volume into the plastic material with the additives. Rotational drive energy imparted to the kneading elements on the extruder screws is dissipated into the plastic material causing heating and inducing mixing of the various additives into a plastic compound melt.
These kneading elements conventionally employ a special cross-section profile which is designed to provide "effective scraping" (very small clearances such as a millimeter or less) between adjacent kneading elements and usually also effective scraping between an outer diameter of the kneading element and the inner wall of the barrel. As a result of this very small clearance geometry, intensive energy is dissipated in the kneading section of the extruder, producing localized extreme heating. This heat energy, if not promptly and continuously removed, results in overheating of the compound melt with possible degradation of the plastic material.
Another problem associated with kneading discs in general is their capability of generating localized high pressures, especially in the vicinity of the kneading disc tip. These localized high pressures result in shaft-deflecting forces which push the screw shafts towards the inner surfaces of the barrel walls, thus accelerating wear of the extruder. From a process point-of-view, such localized high pressures may re-fuse back together and agglomerate solid particles previously broken apart, thereby acting contrary to the objective of obtaining a homogeneous compound melt. Additionally, in such prior machines when dealing with dispersive or extensive mixing, various different fluid particles are being exposed to highly non-uniform kneading shear rates. Consequently, the kneading shearing action must be repeated many times to ensure that all fluid particles have been exposed to equal levels of shear and/or thermal history.